Active-matrix liquid crystal display elements and organic electroluminescence display elements are formed on a glass substrate. The pixels disposed in a matrix on the substrate are controlled by the transistors provided in the vicinity of the pixels. With current technology, however, crystalline semiconductor transistors cannot be formed on the glass substrate, and for this reason thin film transistors formed of an amorphous silicon or polysilicon thin film have been used for the control of the pixels. The thin film transistor has the advantage that it can be formed on a large-area substrate at low cost; however, its smaller mobility compared with crystalline silicon has prevented them from operating at high speed. To overcome such a problem, there have been proposed techniques in which the transistors first are fabricated on a silicon wafer in a large quantity and then cut into individual pieces to be disposed on a substrate.
For example, there has been proposed a technique in which openings for accepting the transistors are formed through a substrate, which then is exposed to a liquid containing monocrystalline silicon transistors dispersed therein so that the transistors are disposed in the openings (see U.S. Pat. No. 6,417,025, Information Display, p. 12-16, 1999). By forming the openings in the shape that matches that of the transistors, the transistors are disposed in a predetermined orientation at predetermined positions on the substrate. As taught in these publications, this technique allows 10,000 transistors, having a size of ten to several hundred micrometers, to be disposed on a substrate measuring 3 inch2.
Further, a fabrication method of a liquid crystal display element in which large numbers of monocrystalline silicon transistors are disposed on a glass substrate has been disclosed (see JP2003-5212A). In this method, a rubber polymer thin film with openings that fit the monocrystalline silicon transistors is formed on a glass substrate, and the glass substrate is exposed to a liquid in which the monocrystalline silicon transistors are dispersed, so that the transistors are disposed on the glass substrate. Providing the openings in the glass substrate requires expensive equipment such as a laser processing device. With this method, however, the openings are not formed directly through the substrate and the transistors can be placed with simple equipment.
There also has been proposed a method in which a substrate having a first mating surface is dispersed in a liquid together with members having a second mating surface of the shape that substantially matches the shape of the first mating surface. By system design, the free energy of the dispersion liquid is minimized in the state where these two mating surfaces are mated to each other, so as to dispose the members on the substrate (see U.S. Pat. No. 6,507,989). For example, a region in a substrate surface is rendered water-repellent to provide the first mating surface, and the other regions in a substrate surface are rendered hydrophilic. In the same manner, one of the surfaces of each member to be disposed on the substrate is rendered water-repellent to provide the second mating surface, and the remaining surfaces of the member, other than the second mating surface, are rendered hydrophilic. Thereafter, an appropriate amount of water-repellent ultraviolet (UV) curable resin is applied to both the first and second mating surfaces, and the substrate and the members are placed in water. By agitation, the first mating surface of the substrate and the second mating surface of the member are joined together via the UV curable resin. By irradiation of the substrate with UV light in water, the resin is cured to fasten the first mating surface of the substrate firmly to the second mating surface of the member. A method also is disclosed in which hexadecane is applied instead of the UV curable resin to the first and second mating surfaces, and the substrate on which the members have been disposed is taken out of water and heated to remove the hexadecane and thereby fasten the first mating surface of the substrate to the second mating surface of each member (Journal of Microelectromechanical Systems, Vol. 10, No 1, 2001).
Meanwhile, with the recent advance in nanotechnology, various ideas have been put forth and research and development has been active on electronic devices using pillar-like members having a diameter smaller than several hundred nanometers (may be referred to as “nanomembers” hereinafter). The pillar-like nanomembers include needle-like nanoparticles, for example, such as carbon nanotubes and semiconductor nanowires. An application of such nanomembers as the constituting members of an electronic circuit (may be referred to simply as “members” hereinafter) is described, for example, by D. Wang, et al., “Germanium nanowire field-effect transistors with SiO2 and high-k HfO2 gate dielectric”, Appl. Phys. Lett. Vol. 83, pp. 2432, 2003, which describes operations of field-effect transistors (FETs) with semiconductor nanowires at ordinary temperature. The field-effect transistors using such nanomembers are fabricated by a coating technique, and as such the fabrication does not require techniques using various kinds of large-scale vacuum equipment as required in conventional thin film techniques. Conceivably, the technique disclosed in this publication has many advantages, including cost reduction.
In order to realize transistor characteristics using nanomembers, the nanomembers need to be disposed in predetermined microscopic regions in a uniaxial orientation. This is necessary because field-effect transistors can be realized by forming a source electrode and a drain electrode at both ends of each pillar-like nanomember respectively. The nanomember has been disposed in a uniaxial orientation. Therefore, one of the big challenges, in order to fabricate the field-effect transistors of a coating type using nanomembers, is the precise control of the orientation and position of the nanomembers in mounting the nanomembers on the substrate. For example, as a method of controlling the orientation and position of the nanomembers, there has been reported a method in which a mold made of polydimethylsiloxane (PDMS) with large numbers of grooves is brought into contact with a substrate surface to form channels for flowing a liquid, and a liquid in which the nanomembers are dispersed flows through the channels to coat the substrate with pillar-like nanomembers in a particular orientation (referred to as “flow method” hereinafter) (see U.S. Pat. No. 6,872,645, Y. Huang, et al., “Directed Assembly of One-Dimensional Nanostructures into Functional Networks,” Science vol. 291, pp. 630, 2001). Further, for example, there has been a report in which a suspension of nanomembers whose surfaces have been rendered hydrophilic by chemical modification is prepared and a substrate whose surface partially has been rendered hydrophilic is brought into contact with the suspension and then separated therefrom to dispose the nanomembers in a certain but limited orientation on the hydrophilic portion of the substrate, by utilizing the liquid/solid/gas interfaces between the substrate, suspension, and atmosphere (see U.S. Pat. No. 6,969,690). As a method of removing the suspension from the substrate, a method has been proposed in which the substrate partially is immersed in the suspension and the solvent of the suspension gradually is evaporated.
The conventional technique in which the substrate is exposed to a transistor-dispersed liquid to set the transistors in the openings of the substrate works under the principle that the transistors that have approached the openings fall therein by the force of gravity when the transistors have the shape that fit the openings. As such, the probability that the transistors in the vicinity of the openings fall into the openings is not 100%. This probability becomes even smaller as the size of the transistors is reduced, because the surface tension that acts on the element surface or the force of liquid flow will increase to be more than the gravitational force acting on the elements. This necessitates that the transistors in the dispersion liquid be provided in greater numbers than the numbers needed for the substrate. Thus, fabrication of a single display element conventionally required greater numbers of transistors than the numbers of transistors actually needed. This has posed the problem of high manufacturing cost. Further, because whether the transistors will fall in the openings is a question of probability, the probability of having an opening with no transistor will not be zero even when the substrate is exposed to the dispersion solution for extended time periods. This necessitated checking for empty openings, requiring additional fabrication steps.
As to the conventional method in which a liquid is disposed on the respective predetermined surfaces (mating surfaces) of the substrate and the member to join the substrate and member at these predetermined mating surfaces in a dispersion liquid (dispersion medium), the method provides an excellent way to dispose the members on the substrate. However, it has difficulties in controlling the amount of liquid disposed on the mating surfaces (Sensor Update, Vol. 13, P3, 2004). Specifically, when the amount of liquid is too small, the contact surface between the substrate and the member will not be completely covered with the liquid and the adhesion is weak. On the other hand, when the amount of liquid is too large, the members will float on the liquid surface and move around, causing a problem that the members will detach from the liquid even with little stirring. Another drawback of this method is the low dispersibility of the members to the dispersion medium due to the two kinds of member surfaces with dissimilar properties (water-repellent mating surface and the hydrophilic surface in the remaining portion). This causes a problem that the members will adhere to the air/liquid interface of the dispersion medium, or agglomerate. This tendency is more pronounced when the members are small. Further, with nano-scale members, it is technically difficult to provide different wettability to different portions of the surface. This has made the placement of the nano-scale members on a substrate difficult. Further, in the conventional examples, the liquids placed on the mating surfaces are all hydrophobic. The adhesion between the substrate and the member is determined by the surface tension of the liquid placed on these surfaces. However, hydrophobic liquids have lower surface tensions compared with hydrophilic liquids such as water and there is only a weak force that binds the substrate and the member together. For this reason, when the substrate with the disposed members is taken out of the dispersion liquid, there are cases where the members come off the substrate (Journal of Microelectromechanical Systems, Vol 10, No 1, 2001).
When the conventional flow method is used to control the orientation and position of the pillar-like nanomembers in mounting these members on a substrate, there are difficulties in stably orienting and positioning the members. Further, since this method uses a mold that regulates the direction of a liquid flow, it requires complicated fabrication steps and therefore complex equipment. Because of this, the method has the problem of high manufacturing cost and poor reproducibility. Further, accurate orientation of the nanomembers is difficult to achieve with the conventional method utilizing the liquid/solid/gas interface. The method also requires strict control in the step of removing the suspension from the substrate. The method therefore requires complex equipment, which poses the problem of high manufacturing cost and poor reproducibility.